In an optical transmission system that transmits data with frames, transmission delay is measured between nodes in order to confirm the reliability of data transmission. It is desirable to perform this kind of transmission delay measurement while an optical transmission system is operating.
There is a method proposed as a technique for measuring transmission delay in optical transmission systems. According to this technique, the transmission device attaches time information to frames and transmits those frames, and the reception device decides the order of processing the frames according to the time information. Also, a propagation delay and transmission intervals are measured by using the time information attached to the frames, and further the guard time is calculated from the propagation delay so as to configure this guard time in each communication path, and thereby the frames are processed in the right order (Japanese Laid-open Patent Publication No. 2007-336191 for example).
However, in the conventional method described above, the transmission device and the reception device must be synchronized to each other in order to measure the transmission delay. That is to say, the times on the clocks in the transmission device and the reception device have to be synchronized to each other.